Electromechanical oscillator with rotary output



C. F'. CLIFFORD Oct. 6,1910 V T Q I .E IEECTR OME CHANICAL OSCILLATOR WITH ROTARY OUTPUT Filed Feb. 19, 1969 W a T A M O ra/io r United States Patent 3,532,912 ELECTROMECHANICAL OSCILLATOR WITH ROTARY OUTPUT Cecil Frank Clifford, Newbridge Works,

Bath, Somerset, England Filed Feb. 19, 1969, Ser. No. 800,552 Claims priority, applicatisolll great Britain, Feb. 20, 1968,

8 Int. Cl. H02k 7/06 U.S. Cl. 310-21 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an electromechanical oscillator of a kind which provides a rotary output.

A magnetic escapement has been known for many years in which an escape wheel is formed with a wavy magnetic track around each of its two faces and an oscillating reed bifurcated at the oscillating end is provided with inturned magnetic poles, the reed being placed so that it straddles the wheel and the two magnetic poles lie close to wavy magnetic tracks around the faces of the escape wheel. Power provided by a spring, an electric motor or other convenient means is applied to the spindle of the escape wheel so that it tends to rotate. The escape wheel spindle is also coupled to the gear train of a timing mechanism which may be part of a timing instrument or may be an ordinary clock. The forced rotation of the escape wheel causes the reed to oscillate at its own natural frequency. Thus the rate at which the escape wheel and the clock train are allowed to escape is determined by the natural frequency of the reed and the number of waves in the magnetic tracks around the escape wheel. In this arrangement the power required to drive the timing train is conveyed mechanically from the driving means to the train and the only power which has to be transmitted through the magnetic coupling between the escape wheel and the oscillating reed is the small amount required to maintain thereed in oscillation.

Electromechanical oscillators have more recently been developed in which either a reed or the two tines of a tuning fork are caused to oscillate and are maintained in continuous oscillation by electrical means, and the oscillating member or members are provided with magnetic poles or end pieces which coact with the wavy magnetic tracks formed on the faces of an escape wheel of the kind used in the magnetic escapement described previously. The oscillation of the reed or tuning fork tines causes the escape wheel to rotate and to drive the escape wheel and the timing train. In this case, however, the power required to maintain the reed or the tuning fork in oscillation is applied by electromagnetic means to the reed or fork, and the whole of the power required to drive the timing train must be transmitted via the magnetic coupling from the oscillating member or members to the escape wheel. Moreover, the wheel ceases to be an escape wheel in the accepted sense, and becomes a driving wheel for the timing train. It will therefore be 3,532,912 Patented Oct. 6, 1970 referred to as a driving wheel throughout the remainder of this specification.

There is a continuing requirement that electromechanical oscillators should be made as small as possible and that they should draw as little electrical power from the electrical source, usually a battery, as possible. Hence it is of the utmost importance that the transmission of power from the oscillating member or members to the driving wheel should be as eflicient as possible. The principal object of the invention is to provide an electromechanical oscillator of the kind described in which the efliciency of power transmission via the magnetic coupling between the oscillating member or members and the driving wheel is exceptionally high.

The escape wheels for the magnetic escapement described earlier are usually made from sheet magnetic material and are shaped by punching so as to form a number of parallel-sided radially projecting teeth around the outer periphery, and a number of apertures arranged in a circle are punched in the material inside the teeth, the centre line of each aperture lying on the radial centre line of one of the teeth, so that the material left between the teeth and the apertures defines the wavy magnetic track. Naturally, such a track formed on each face of the wheel.

In carrying out experiments with this type ofwheel it has been found that for maximum efficiency of the drive from the oscillating member or members there should be a certain relationship between the circumferential width of the teeth and circumferentnal width of the gaps between them, and the circumferential width of the apertures and the circumferential width of the webs or spokes of material between adjacent apertures. For convenience in explaining the invention the circumferential width of the gap between adjaceint teeth, and the circumferentail width of the apertures will both be referred to as the aperture width and the circumferential width of the teeth and the circumferential width of the web of material between adjacent holes will be referred to as the spoke width. Thus the driving whel will have outer and inner spokes, and outer and inner apertures.

Since the inner apertures are arranged on a circle of smaller diameter than the outer apertures it will be evident that if the inner and outer spokes are of the same circumferential width then the inner and outer apertures cannot be of the same circumferential width, and the aperture width used to define the invention is the mean of the two. It has been found that for eflicient transmission of power from the oscillating member or members to the driving wheel the ratio of mean aperture width (as herein defined) to spoke width (as herein defined) should be at least 2. This in itself is perhaps not very surprising. What is surprising, however, is that as this ratio is increased above 2 the torque delivered at the driving wheel spindle increases until the ratio is well above 3. The conditions which obtain if the ratio is substantially above 4 have not been fully established at the time of writing this specification but it is logical to assume that there is an upper limit above which the torque will begin to fall off again. In establishing these figures it is, of course, necessary to ensure that the oscillating member or members have suflicient energy to sustain the comparatively heavy torque which is obtainable at the driving wheel spindle.

The invention consists of an electrochemical oscillator with rotary output comprising at least one mechanically oscillating member which is maintained in oscillation by electrical means, a driving wheel having radially projecting outer spokes separated by outer apertures, the driving wheel also having inner spokes separated by inner apertures, whereby the boundaries of the apertures define wavy magnetic tracks on both faces of the driving wheel, the

inner and outer spokes being of equal circumferential width at their narrowest points, the ratio between the mean circumferential aperture width and the circumferential spoke width being at least 2, and a magnetic member on at least one oscillating member coating with at least one wavy magnetic tracks so that the driving wheel is driven by the oscillating member.

Preferably the said ratio is above 2 and does not exced 4.

In a favoured form of driving wheel the roots of the outer spokes are joined by circular arcs, and at least the portions of the inner apertures adjacent the outer spokes are bounded by circular arcs.

Selected embodiments of the invention will now be described with reference to the accompanying drawings in which FIG. 1 shows a part of an escape wheel which conforms to the invention;

FIG. 2 shows a family of curves illustrating the results obtained with various types of driving wheel;

FIG. 3 is a curve illustrating the results obtained with different mean aperture width/spoke width ratios; and

FIG. 4 is a diagram of a circuit for maintaining a reed in mechanical oscillation by electrical means.

Referring to the drawings, FIG. 1 shows a part of a driving wheel consisting of a series of outer spokes 11 which have parallel flanks, each spoke being centered about a line radial to the axis of the wheel. Adjacent spokes are separated by spaces bounded by circular arc portions 12 having a radius R, which are referred to herein as outer apertures. Lying on the radial centre line of each tooth is an inner aperture 13. As shown the inner apertures 13 are circular, but it is only necessary to provide that those parts of the inner apertures which are adjacent the outer spokes and the outer apertures are bounded by circular arcs. The circular arcs joining the roots of the outer spokes and bounding the outer parts of the inner apertures define a wavy magnetic track whose centre is indicated by the dotted line 14. There is, of course, a wavy magnetic track on each face of the wheel.

The driving wheel shown in FIG. 1 is part of an electromechanical oscillator which compirses one or more oscillating members, which may compirse either a single reed or the tines of a tuning fork. Where a single reed is employed it may be provided at its end with a small magnet and where a tuning fork is employed such magnets may be mounted on both the tines, the magnets being separated by a distance equal to the diameter of the pitch circle of the wavy magnetic track 14, the track having an even number of waves. The reed or the tuning fork, as the case may be, is maintained in continuous oscillation by means of an electrical maintaining circuit, which is usually an amplifier, in the well known manner, and by virtue of the magnetic coupling between the magnet at the end of the reed, or the magnets at the ends of the tines, the driving wheel is caused to rotate. The spindle on which the wheel is mounted is coupled to a timing train (not shown) which may be a part of a clock or may be the mechanism of a timing device of some other sort. The rate at which the driving wheel of FIG. 1 rotates is governed by the frequency of oscillation of the reed or the tuning fork and the number of waves in the magnetic track 14.

Since the electrical power to maintain the oscillating member or members in oscillation is supplied by the am plifier it will be clear that the whole of the driving power for the clock or timing device is drawn from the electrical supply which energizes the amplifier. Moreover, the whole of the power required to drive the timing train must be transmitted by the magnetic coupling between the oscillating member or members and the driving wheel. Accordingly it is very important that this coupling should be as efiicient as possible to provide the maximum torque at the driving wheel.

As the result of the rsearch work previously referred to on the problem of providing an eflicient magnetic drive to the timing wheel it has been found that the magnetic coupling becomes most efiioient if there is a definite relationship between the mean circumferential aperture width, that is, the mean of the dimensions indicated at a and b in FIG. 1, and the spoke width, that is, the dimensions indicated at w in FIG. 1, and that this ratio should lie between 2 and 4, and preferably by nearer 3. That is to say, if the ratio between mean circumferential aperture width and spoke width is denoted by the letter r then mean circumferential aperture width spoke width and does not exceed 4. Since the inner and outer spoke Widths are equal, as denoted by the common dimension w, the dimension (1 must be larger than the dimension [7 and for the purpose of defining the ratio the mean of the two dimensions is taken, that is, a+b/2.

It follows that =at least 2 Having defined certain characteristics of the wavy magnetic tracks the size of the cooperating pole or pole piece on the or each oscillating element must also be defined. It is possible to use a magnetic pole of circular crosssection, as indicated in dotted lines at 15, but it is preferable to use an elongate (that is, oval or rectangular) magnetic pole. For example, if, in a given arrangement, with a pole 15 of circular section, the torque transmitted is 1 unit, then by substituting an elliptical member having the same circumferential width as the element 15 but having a radial width such that the major axis of the ellipse is twice the minor axis, the torque transmitted to the wheel rises to 1.6 units and if a rectangular element is substituted in which the long side of the rectangle is double the short side then the torque transmitted rises to 1.8 units. These results are, of course, only obtainable if the oscillating member or members have suffioient margin of energy to supply energy necessary to sustain the torques mentioned above. The elongate shape or pole is disclosed in our British Pat. No. 1,128,394.

As noted above, the ratio r should be at least 2 to secure a high torque at the spindle of the driving wheel, and not greater than 4.

FIG. 2 shows the results of tests made to establish the optimum ratio 1'. All these curves show the results of tests made with wheels having an outside diameter of 15 millimetres, made from sheet metal of the nickel-iron alloy known by the registered trademark Mumetal and 0.25 millimetres thick. The oscillator frequency was 300 Hz. and the torque recorded is that obtainable at a spindle rotating at a speed of onerevolution per minute. The horizontal scale represents the amplitude of oscillation in millimetres and inches 'while the vertical scale represents the torque obtainable at a speed of 1 r.p.m. which is the speed of the seconds handin a clock.

Tests were made with driving wheels having 40 outer spokes and these are indicated by curves 15, 16, 17 and 18. In all these four curves the ratio r is 1.37 but the gap width, that is, the distances between the pole face of the oscillating member and the adjacent face of the wheel, varied from 0.2 millimetre in curve 15 through 0.15 millimetre in curve 16 and 0.1 millimetre in curve 17 to 0.05 millimetre in curve 18. As is to be expected the torque transmitted increases as the gap is reduced, for a given ratio r. Curve 19 shows the results obtained with a wheel with 30 outer spokes and a 0.1 millimetre gap where the ratio is 1.0. Cunve 20 shows the results obtained with a wheel with 20 spokes and a 0.1 millimetre gap where the ratio r is 3.0. Comparing the curves in which the gap is 0.1 millimetre it will be seen that curve 19 shows the lowest torques with a ratio of 1.0. Curve 17 gives an improved result where the ratio r is equal to 1.37 and curve 20 shows a considerable improvement Where the .ratio r is increased to 3.0 From a consideration of these curves a curve 21 was drawn showing the results which would be expected from a wheel with 30 outer spokes and a 0.1 millimetre gap, with a ratio r of 2.0.

It has been found that a reduction of the gap between the magnet pole and the :wheel face provides an improvement in performance, as may be expected and as is shown by the curves. However, the more spectacular feature of the curves is the extent of improvement brought about by an increase in the ratio r in increasing the torque obtainable from the driving Wheel There must be suflicient energy in the oscillating member to sustain the torque and this is, of course, a function of the amount of energy supplied to the oscillating member by the amplifier, and hence by the power supply.

FIG. 3 shows the torque which is obtainable from wheels with 40 outer spokes using ratios ranging from 0.5 to 3.0 with a 0.1 millimetre air gap and with the oscillating member oscillating with an amplitude of 0.5 millimetre. As will be evident, the torque obtainable rises linearly with the ratio r and from this the law relating to that particular curve may be deduced (writing T for torque) as T=4r-2. Very roughly it is found that for the wheel with 40 outer spokes. These results show clearly that while an improvement in torque can be pro duced by making the gap between the magnetic member and the face of the wheel as small as possible, the major influence is the ratio r and this is shown quite dramatically in FIG. 3.

FIG. 4 shows a circuit for maintaining a reed in mechanical oscillation by electrical means. The reed 22 is carried on a fixed support 23. Two coils, respectively 24 and 25, are each provided with a magnetic core, respectively 26 and 27. One end of the winding of coil 24 is connected to the input of an amplifier 28 and the other end is connected to an earth or ground line 29. The output of amplifier 28 is connected by line 30 to one end of the winding of coil and the other end thereof is connected to the earth or ground line 29. This circuit is completely conventional and a similar circuit may be used in conjunction with a tuning fork oscillator. The reed 22 is shown having a magnetic member 31 at its oscillating end co-operating with the wavy magnetic track of a driving wheel 32, whereby the driving wheel is driven.

I claim:

1. An electromechanical oscillator with rotary output comprising at least one mechanically oscillating member which is maintained in oscillation by electrical means, a driving wheel having radially projecting outer spokes separated by outer apertures, the drving wheel also having inner spokes separated by inner apertures, whereby the boundaries of the apertures define wavy magnetic tracks on both faces of the driving wheel, the inner and outer spokes being of equal circumferential width at their narrowest points, the ratio between the means circumfrential aperture width and the circumferential spoke width being at least 2, and a magnetic member on at least one oscillating member coacting with at least one wavy magnetic track so that the driving wheel is driven by the oscillating member.

2. An oscillator as claimed in claim 1 in which the driving wheel is so shaped that the roots of the outer spokes are joined by circular arcs, and at least the portions of the inner apertures adjacent the outer spokes are bounded by circular arcs.

3. An oscillator as claimed in claim 1 in which the said ratio is between 2 and 4.

4. An oscillator as claimed in claim 1 in which the inner apertures are in the form of circular holes set on a pitch circle centered on the driving wheel axis.

5. An oscillator as claimed in claim 1 in which the or each magnetic member is circular in cross section and has a diameter substantially equal to the width of the wavy magnetic track at its narrowest points.

6. An oscillator as claimed in claim 1 in which the or each magnetic member is elongate in cross section.

References Cited UNITED STATES PATENTS 2,946,183 7/1960 Clifford 74l.5 X 3,148,497 9/1964 Clifford et al 310--21 X 3,277,644 10/1966 Nomura et al. 58-23 DONOVAN F. DUGGAN, Primary Examiner 

